Generally speaking, with the term “Medium Voltages” (briefly, MV) it is intended a range of voltages of the order of the tens of KVolts. For example, the MV range may extend from 1 KVolts to 52 KVolts.
Usually, the power cables used for conveying or supplying electrical power at these voltage levels comprise a plurality of components. Starting from the inside of the cable and proceeding toward the outside thereof, a power cable typically includes a metal conductor, an inner semiconductive layer, an insulating layer, an outer semiconductive layer, a metal screen—usually made of aluminum, lead or copper—and an external—typically, polymeric—cable sheath.
The structure, the material and the size of these components vary according to the particular application for which the power cable is intended and the expected environmental conditions to which the cable is subjected. For example, the cross-sectional size of the metal conductor is mainly determined by the current-carrying capacity of the cable, the thickness of the semiconductive and insulating layers is mainly determined by the value of the working voltage, while the shape and composition of the cable sheath is mainly determined by the environmental conditions to which the cable is subjected.
When two cable lengths have to be joined together, a construction usually called “cable joint” is provided, to get the electric connection and to restore the insulation and protection of the cable.
The discussion below is made with specific reference to cable joints, but it can apply to other conditions, such as cable terminations, where similar problems arise. Moreover, even if reference will be made to power cables for medium voltage applications, similar considerations apply to power cables designed for operating within different voltage ranges, such as those corresponding to low and high voltage applications.
For the purposes of the present invention, by “cable joint” term is meant any circumstance, in which the cable sheath and possibly underlying layers are exposed to provide access to the parts of the cable construction, such in cable connection assembly as cable joints, cable terminations, branch cable joints, stop-ends and the like. The assembly is used to restore properties of the electrical line, said assembly, in particular, including an external sheath to be applied over the area of removal of the cable sheath.
In the following, unless differently specific, the term “cable joint” is meant to encompass also these other components showing the same problems and getting benefit from the same solution.
In order to connect the ends of two power cables for establishing a common electrical connection, such ends are firstly processed so as to expose, over a portion of defined length, each one of the components forming both the cables. Then, the exposed metal conductors of the two power cables are connected to each other, for example through soldering or by means of a suitable metallic clamp.
In order to restore the continuity among the other components of the two cables, a suitable joint element is positioned on the zone wherein the metal conductors are connected. Usually, a joint element of this type comprises a sleeve element adapted to be fitted on the two ends of the power cables. Such sleeve element has a generally cylindrical central portion, with two frustoconical ends.
The sleeve element comprises a plurality of superimposed layers. For example, a typical sleeve element may comprise a stress control layer made of material with a high dielectric constant, an insulating layer of insulating material covering the stress control layer, and a layer of semiconductive material covering the insulating layer.
A sleeve element of the so-called cold-retractable type is generally supplied fitted, in an elastically-dilated condition, on a hollow tubular support made of rigid plastic material. Such tubular-supported sleeve element is fitted on one of the two power cables before the formation of the connection between the metal conductors.
The tubular support may be made using different methods which allow the removal thereof once the sleeve element has been correctly positioned. For example, the tubular support may be made in the form of a helix so that, when a pulling force is exerted on a free end portion of said strip-like element, the tubular support is caused to collapse over the cable ends. In so doing, the sleeve element elastically contracts, clamping over the cable sections in the joining zone.
Sleeve elements of the so-called heat-shrinkable type are also known, which are formed by heat-shrinkable materials.
Other types of sleeve elements are known, such as the so-called slip-on sleeves (formed by pre-molded components fitted on the cables using proper lubricants), the so-called taped sleeves (whose components are assembled using insulating, semiconductive and/or high permittivity tapes), and the resin-based sleeves.
A joint element typically further comprises a joint shield configured to restore the metal screen over the portions of the two power cables which have been exposed. For example, a tin-plated copper strip may be applied starting from the exposed metal screen portion of the first cable and ending on the exposed metal screen of the second cable.
In the case where the joining operation is performed between two sections of electrical cable of the multi-pole-for example double-pole or triple-pole type, the procedure described hitherto is repeated for each single phase of each cable.
Usually, a joint element as defined above further comprises an external sheath suitable for restoring over the exposed portions of the two power cables the mechanical protection offered by the external cable sheaths. Such external sheath of the joint is usually made of a polymeric material and is fitted on the outside surface of the joint shield, so as to protect the underlying layers from coming into contact with the outer environment (e.g., moisture and/or water, etc. . . . ).
Preferably, the joint shield is usually biased to the ground voltage through a proper biasing connector and attached to a surface of the exposed metal screen portion of one of the two cables. Since such exposed metal screen is electrically connected to the joint shield, by grounding the exposed metal screen portion of a cable through such biasing connector, the joint shield itself results to be accordingly grounded.
Known biasing connectors generally comprise a conductive tape connected to an end portion configured to allow the biasing connector to be firmly fixed on the exposed metal screen of one of the power cables; for example, such end portion is adapted to mechanically cooperate with a surface of the metal screen by applying a radial tightening thereto. The conductive tape is made of a braid of woven metallic wires, usually made of tinned copper, which extends from a first end soldered to the end portion to a second end comprising a socket connector adapted to be fastened to a terminal providing the ground voltage. In this way, the joint shield can be grounded through the conductive path formed by the conductive tape, the end portion and the metal screen of the cable.
The use of the conductive tape made of a braid of woven metallic wires has been considered important because its flexibility allowed the tape to mate precisely with the surface of the cable sheath, thereby minimizing the deformation of the external sheath, possible source of water penetration.
In order to prevent the occurrence of mechanical faults in the conductive tapes and for increasing the operative life thereof, particular care has to be employed for protecting the braid of woven metallic wires from possible water and humidity infiltrations.
Moreover, since the conductive tape of the biasing connector has to pass between the external sheath of the joint element and the cable sheath, in order to be capable of reaching the terminal providing the ground element, particular care has also to be employed for avoiding that water and humidity infiltrate within the interior of the joint element through such opening.
For these purposes, the water and humidity resistance of the conductive tape and of the joint element is improved by coating the conductive tape that protrudes out of the joint element with a proper protective sheath. Generally said protective sheath covers both the two surfaces of the braid of woven metallic wires of the conductive tape.